The present invention relates to a carrier for the development of electrostatic latent image for use in electrophotography, electrostatic recording, electrostatic printing, etc. The invention also relates to a developer for the development of electrostatic latent image comprising the carrier and to an image forming method using the developer.
A method which comprises making image data visible via electrostatic image such as electrophotography has wide application at present. Electrophotography has heretofore normally employed a process which comprises forming an electrostatic latent image on a photoreceptor or electrostatic recording material by various means, and then allowing electroscopic particulate material called toner to be attached to the electrostatic latent image to develop and make the electrostatic latent image visible. The toners to be used for this purpose can be roughly divided into two groups. That is, one of them is a two-component developer arranged such that a supporting particle such as carrier and a particulate toner are mixing by each other to cause electrification so that the toner is provided with a proper amount of positive or negative charge. The other of them is one-component developer comprising a toner alone such as magnetic toner. In particular, the two-component developer can provide the carrier itself with functions such as agitation, conveyance and electrification, making it possible to separate functions required for developer. Thus, the two-component developer has found wide application because of easy design.
However, the two-component developer for use in frictional electrification is readily subject to change of electrification level when affected by environmental change. In general, high electrification can easily occur under low temperature and low humidity conditions while low electrification can easily occur under high temperature and high humidity conditions. Thus, the two-component developer is disadvantageous in that environmental change leads to the reduction of density in the case of high electrification or to the occurrence of fog in the case of low electrification.
Processes for the preparation of a toner can be roughly divided into two groups, i.e., a process involving conventional dry melt-knead grinding method and a wet process involving granulation of a toner in a liquid. The wet process has been given more attention from the standpoint of reduction of diameter of toner, sharpness of particle size distribution, freedom of shape control, reduction of energy cost of production, etc. However, since granulation is effected in a liquid, a hydrophilic group can be easily left on the surface of the toner, causing moistening under high humidity conditions and hence deteriorating chargeability. Therefore, the conventional developer comprising a toner obtained by such a wet process is disadvantageous in that it exhibits deteriorated chargeability under high humidity conditions.
The change of electrification (environmental dependence) of the carrier becomes larger as the chargeability of the coating resin increases. For example, a carrier comprising a polymethyl methacrylate as a coating resin has a higher electrification level but exhibits a higher environmental dependence than that comprising a polystyrene as a coating resin. In other words, a resin material containing a group having a high polarity has a high electrification level but exhibits a high environmental dependence while a resin material having a low polarity has a low environmental dependence but has a lower electrification level. Thus, it is difficult to allow both the toner and carrier to keep desired electrification level and provide chargeability which has little environmental dependence.
A carrier is required to have a good electrification-maintaining property. In other words, the carrier is required to maintain desired electrification level over an extended period of time. The deterioration of carrier has been heretofore considered attributed to the following two causes.
Firstly, the deterioration of carrier is attributed to the fixing of toner components to the surface of the carrier coating resin. When a toner and external additives having a polarity opposite to that of the carrier are attached to the carrier, the chargeability of the carrier is deteriorated. Further, since the toner is insulating, the carrier to which the toner has been attached shows a raised resistivity.
Secondly, when the coated carrier is subject to stress with time, the coating resin is peeled off to reduce the amount of the coating resin. As a result, the carrier exhibits deteriorated chargeability. Further, when the core material is exposed, the carrier exhibits deteriorated chargeability and lowered resistivity.
In order to solve these problems, it has been proposed to use a fluororesin or silicone resin as coating resin so that the surface energy of the coating resin can be reduced to prevent pollution of the surface of carrier or raise the strength of the coating resin so that the peeling off or loss of the coat layer can be prevented. However, since such a low surface energy material has a deteriorated adhesion to core material, it is very difficult to provide the coating resin with both pollution resistance and peeling resistance.
JP-A-2-114271 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposed a carrier coated with a copolymer of (meth)acrylic acid ester monomer with a monomer having a carboxyl group for the purpose of enhancing the adhesion of coating resin to improve the electrification maintaining property. The carboxyl group to be used in this proposal is effective to enhance the negative chargeability of carrier and hence the adhesion to core material such as metal and durability. However, the carboxyl group, which is a hydrophilic group, comes up to the surface of the coating resin to exhibit hygroscopicity, deteriorating chargeability and hence making it easy for the carrier to be contaminated to disadvantage.
JP-A-61-80161, JP-A-61-80162, and JP-A-61-80163 proposed that when the surface of a carrier core material is coated with a copolymer of nitrogen-containing fluorinated alkyl (meth)acrylate and vinyl monomer or a copolymer of fluorinated alkyl (meth)acrylate and nitrogen-containing vinyl monomer, a coated carrier having a relatively prolonged life which can difficultly be contaminated by toner or external additives can be provided. However, this approach is disadvantageous in that since a fluororesin has an insufficient chargeability, the resulting carrier has a raised environmental dependence when copolymerized with a nitrogen-containing vinyl monomer or methyl methacrylate ester monomer having a highly chargeable polar group.
Paying attention to methacrylic acid ester monomer for the purpose of solving the problem of environmental dependence, JP-A-59-104664 proposed a carrier coated with a methacrylic acid cycloalkyl ester polymer having an excellent moisture resistance. However, this carrier has a low electrification level, high brittleness and a deteriorated adhesion to core material as compared with that coated with methyl methacrylate.
In order to solve the foregoing problem, JP-A-7-114219 proposed a carrier coated with a copolymer of methacrylic acid cycloalkyl ester and methyl methacrylate. However, cycloalkyl ester is brittle and thus can be easily peeled off. Further, when the composition ratio of methyl methacrylate is increased, the resulting environmental stability is deteriorated, making it impossible to accomplish desired durability and environmental dependence at the same time.
Further, JP-A-60-66264 proposed a carrier coated with a copolymer containing tertiary butyl acrylate component and methyl acrylate component. The incorporation of two or more such (meth)acrylic acid esters makes it easier to control chargeability than coated carrier comprising a homopolymer alone. However, since this coating exhibits deteriorated adhesion to core material and becomes brittle similarly to cycloalkyl ester, the resulting coated carrier exhibits deteriorated durability and leaves something to be desired in electrification-maintaining property.
It is thus very difficult to satisfy the reliability requirements, i.e., electrification level, electrification-maintaining property, environmental stability, pollution resistance and durability, of carrier at the same time.
It is therefore an object of the invention to provide a carrier for the development of an electrostatic latent image excellent in electrostatic property, electrification-maintaining property, environmental stability, transferability, durability and pollution resistance.
Another object of the invention is to provide a developer comprising the carrier.
A still other object of the invention is to provide a method for forming a high quality image.
Other objects and effects of the present invention will become apparent from the following description.
The above-described objects of the present invention have been achieved by providing the following carriers, developers, and image forming methods.
1. A carrier for the development of electrostatic latent image comprising a core material and a resin for coating the core material therewith, wherein the coating resin comprises a copolymer of at least four kinds of monomers including:
(1) a monomer containing a carboxyl group;
(2) an acrylic acid alkylester monomer or methacrylic acid alkylester monomer containing a C1-C3 linear alkyl group;
(3) an acrylic acid alkylester monomer or methacrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group; and
(4) a monomer containing fluorine.
2. The carrier for the development of electrostatic latent image according to Clause 1, wherein the proportion of the carboxyl group-containing monomer is from 0.1 to 20.0 parts by weight per 100 parts by weight of the copolymer, the proportion of the fluorine-containing monomer is from 0.1 to 60.0 parts by weight per 100 parts by weight of the copolymer, the balance is a (meth)acrylic acid alkylester monomer, and the ratio of the acrylic acid alkylester monomer or methacrylic acid alkylester monomer containing a C1-C3 linear alkyl group (2) to the acrylic acid alkylester monomer or methacrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group (3) is from 10:90 to 90:10.
3. The carrier for the development of electrostatic latent image according to Clause 1 or 2, wherein the fluorine-containing monomer (4) is a fluoroalkyl (meth)acrylate monomer.
4. The carrier for the development of electrostatic latent image according to any one of Clauses 1 to 3, wherein the core material has a shape factor SF1 of not greater than 125, a true specific gravity of from 3 to 5 g/cm3 and a saturated magnetization of not smaller than 40 emu/g.
5. The carrier for the development of electrostatic latent image according to any one of Clauses 1 to 4, wherein the core material is a magnetic powder-dispersed spherical core material.
6. A two-component developer for the development of electrostatic latent image comprising a toner and a carrier, wherein the toner comprises a binder resin containing a carboxyl group-containing monomer and the carrier is according to any one of Clauses 1 to 5.
7. The two-component developer for the development of electrostatic latent image according to Clause 6, wherein the content of the carboxyl group-containing monomer is from 0.05 to 5 parts by weight per 100 parts by weight of the binder resin.
8. The two-component developer for the development of electrostatic latent image according to Clause 6 or 7, wherein the toner is produced by a wet process.
9. An image formation method comprising:
forming an electrostatic latent image on an electrostatic latent image holding member;
developing the electrostatic latent image with a developer on a developer holding member to form a toner image;
transferring the toner image to a transfer material; and
fixing the toner image on the transfer material,
wherein the developer contains a carrier according to any one of Clauses 1 to 5.
10. An image formation method comprising:
forming an electrostatic latent image on an electrostatic latent image holding member;
developing the electrostatic latent image with a developer on a developer holding member to form a toner image;
transferring the toner image to a transfer material; and
fixing the toner image on the transfer material,
wherein the developer is according to any one of Clauses 6 to 8.
11. The image formation method according to Clause 9 or 10, wherein the developer holding member has a fine unevenness having Rz of from 15 to 25 xcexcm and Ra of from 1 to 5 xcexcm.
The present invention will be further described hereinafter.
The inventors made extensive studies of solution to the foregoing problems. As a result, it was found that the use of a copolymer comprising in combination specific monomers as a coating resin for carrier makes it possible to provide a carrier for the development of electrostatic latent image which exhibits an excellent chargeability under high humidity conditions, less rise in electrification under low humidity conditions and excellent electrification-maintaining property, is not subject to release of coat layer and can hardly attract toner or external additives and the coating of a specific core material with the foregoing coating resin makes it possible to further improve the electrification-maintaining property of carrier. It was also found that the use of a specific toner with the foregoing carrier makes it possible to improve transferability in addition to the foregoing properties and hence form a high quality image.
The coating resin for carrier to be used herein is obtained by the copolymerization of at least four kinds of monomers including: a monomer containing a carboxyl group; a (meth)acrylic acid alkylester monomer containing a C1-C3 linear alkyl group; a (meth)acrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group; and a monomer containing fluorine.
In the coating resin of the invention, the carboxyl group-containing monomer is effective to maintain electrification level and improve the adhesivity of the coating resin to the core material, securing desired durability. However, since the coating resin comprising a carboxyl group-containing monomer is hydrophilic and highly hygroscopic, it is disadvantageous in that the coating resin exhibits a deteriorated chargeability under high humidity. It was then found that the incorporation of a (meth)acrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group is effective to eliminate the foregoing disadvantage and hence improve environmental stability. In other words, the foregoing (meth)acrylic acid alkylester monomer containing a linear or branched alkyl group having many carbon atoms is hydrophobic and thus exhibits no deterioration in chargeability under high humidity conditions. The (meth)acrylic acid alkylester monomer containing a linear alkyl group having many carbon atoms exhibits a lowered glass transition point. On the contrary, the (meth)acrylic acid alkylester monomer containing a branched alkyl group exhibits a raised glass transition point and thus is effective to provide the carrier with desired fluidity. However, these (meth)acrylic acid alkylester monomers can deteriorate the strength of the film and thus cause malelectrification when the toner concentration is high. In order to eliminate this defect, the incorporation of a (meth)acrylic acid alkylester monomer containing a C1-C3 linear alkyl group is effective. This (meth)acrylic acid alkylester monomer has a high environmental dependence. However, the combined use of this (meth)acrylic acid alkylester monomer and the (meth)acrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group makes it possible to eliminate this defect. When the carboxyl group-containing monomer is used, the resulting carrier exhibits a reduced contact angle with respect to water and thus is more subject to contamination by toner or external additives. It was then further found that the incorporation of a fluorine-containing monomer makes it possible to enhance the contact angle of the coating resin and hence improve pollution resistance and maintain desired chargeability over an extended period of time.
These monomers can be incorporated in the same copolymer to secure the foregoing properties. As the copolymerization method there may be used a polymerization method such as random copolymerization and graft copolymerization. The graft copolymerization method is advantageous in that the desired function can be easily exhibited, the adhesion to the core material can be easily improved and Tg of the coating resin can be easily enhanced. However, even if homopolymers of these monomers are blended, the resulting coating resin exhibits deteriorated compatibility, making it impossible to obtain the foregoing properties.
Examples of the carboxyl group-containing monomer employable herein include unsaturated carboxylic acids such as (meth)acrylic acid, vinylacetic acid, allylacetic acid and 10-undecenacetic acid, styrene derivatives such as carboxylstyrene, and compounds having two carboxyl groups such as p-carboxylstyrene.
The amount of the carboxyl group-containing monomer to be incorporated is preferably from 0.1 to 15.0% by weight, more preferably from 0.5 to 10.0% by weight based on the total amount of the monomers constituting the coating resin from the standpoint of adhesion and environmental stability of coating resin. When the amount of the carboxyl group-containing monomer to be incorporated falls below 0.1% by weight, the electrification level is insufficient, deteriorating the adhesion of the coating resin to the core material and hence making it impossible to secure desired durability. On the contrary, when the amount of the carboxyl group-containing monomer to be incorporated exceeds 15% by weight, the resulting coating resin exhibits a raised viscosity that makes it difficult to form a uniform film on the core material and hence impairs the environmental stability of the coating resin.
Examples of the (meth)acrylic acid alkylester monomer containing a C1-C3 linear alkyl group employable herein include methyl (meth)acrylate, ethyl (meth)acrylate, and n-propyl (meth)acrylate.
Examples of the (meth)acrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group employable herein include isopropyl (meth)acrylate, n-butyl (meth)acrylate, tertiary butyl (meth)acrylate, isobutyl (meth)acrylate, tertiary pentyl (meth)acrylate, n-pentyl (meth)acrylate, isopentyl (meth)acrylate, n-hexyl (meth)acrylate, isohexyl (meth)acrylate, and cyclohexyl (meth)acrylate.
The amount of the (meth)acrylic acid alkylester monomer to be used herein is the balance except the amount of the previously mentioned carboxyl group-containing monomer, the fluorine-containing monomer described later and additive monomers, if added to the four monomers. The ratio of the (meth)acrylic acid alkylester monomer containing a C1-C3 linear alkyl group to the (meth)acrylic acid alkylester monomer containing a C4-C10 linear alkyl group or C3-C10 branched alkyl group is adjusted to from 10:90 to 90:10. In this arrangement, the foregoing chargeability, film strength and fluidity can be well balanced. The preferred range of monomer ratio is from 20:80 to 80:20.
Preferred examples of the fluorine-containing monomer employable herein include fluoroalkyl (meth)acrylate monomers containing fluorine such as tetrafluoropropyl methacrylate, pentafluoropropyl methacrylate, octafluoropentyl methacrylate, perfluorooctylethyl methacrylate and trifluoroethyl methacrylate.
The amount of the fluorine-containing monomer to be used herein is from 0.1 to 60.0% by weight, preferably from 0.5 to 50.0% by weight based on the total weight of the monomers constituting the coating resin. When the amount of the fluorine-containing monomer falls below 0.1% by weight, desired pollution resistance cannot be secured. On the contrary, when the amount of the fluorine-containing monomer exceeds 60.0% by weight, the adhesion of the coating resin to the core material is deteriorated and the chargeability of carrier is deteriorated to disadvantage.
As the method for the copolymerization of these monomers there may be used random copolymerization method, graft copolymerization method or the like. The graft copolymerization method is advantageous in that the desired function can be easily exhibited, the adhesion to the core material can be easily improved and the glass transition point of the coating resin can be easily enhanced.
The carrier core material to be used herein is not specifically limited. In practice, however, a magnetic metal such as iron, steel, nickel and cobalt, a magnetic oxide such as ferrite and magnetite, glass bead, etc. can be used. Among these materials, the magnetic materials are preferably used when development is accomplished by magnetic brush method. The volume-average particle diameter of the carrier core material is from 10 xcexcm to 150 xcexcm, preferably from 20 xcexcm to 100 xcexcm. The true specific gravity of the carrier core material is from about 4 to 6 g/cm3.
As the carrier core material there may be alternatively used a spherical core having a magnetic powder dispersed in a resin. Since this spherical core has a small specific gravity, it is advantageous in that stress with respect to toner and carrier can be inhibited. Thus, this spherical core can be used in combination with the foregoing coating resin to secure desired electrification-maintaining property and environmental stability. Examples of the resin to be used in the magnetic powder-dispersed carrier core include crosslinked resins such as phenolic resin and melamine resin, and thermoplastic resins such as polyethylene and polymethyl methacryate.
As the spherical core there may be used a material having the same average particle diameter as that of the foregoing carrier core material. The shape factor SF 1 of the spherical core is preferably not greater than 125. The true specific gravity of the spherical core is preferably from 3 to 5 g/cm3. The saturated magnetization of the spherical core is preferably not smaller than 40 emu/g.
For the measurement of shape factor SF1, optical microscopic image of spherical core sprayed on a slide glass is taken into a luzex image analyzer via a video camera. One hundred or more spherical core particles were then measured for maximum length (ML) and projected area (A). The measured values of ML2/Axc3x97100xc3x97xcfx80/4 were averaged to determine shape factor SF1.
The coated amount of the coating resin for carrier of the invention is preferably from 0.05 to 5.0% by weight so that image quality, secondary hindrance and chargeability can be well balanced. In order to apply the coating resin to the electrification-imparting member, the coated amount and coating method can be properly predetermined to obtain a proper thickness taking into account charged amount and electrification-maintaining property.
The solvent to be used in the starting material solution from which the resin coat layer is formed is not specifically limited so far as it can dissolve the matrix resin therein. Examples of the solvent employable herein include aromatic hydrocarbons such as xylene and toluene, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran and dioxane, and halides such as chloroform and carbon tetrachloride.
Representative examples of the method for forming the coating resin on the surface of the carrier core material or electrification-imparting member include a dipping method involving the dipping of carrier core material powder or electrification-imparting member in a starting material solution for forming a resin coat layer (solvent contains a proper amount of matrix resin, particulate resin, electrically-conductive particulate material, etc.), a spraying method involving the spraying of a resin coat layer-forming solution onto the surface of the carrier core material or electrification-imparting member, a fluidized bed method which comprises spraying a resin coat layer-forming solution onto a carrier coat material being suspended on flowing air, and a kneader coating method which comprises mixing a carrier core material with a resin coat layer-forming solution in a kneader, and then removing the solvent therefrom. However, the present invention is not limited to methods involving the use of solution. Depending on the carrier core material or electrification-imparting member to which the coating resin is applied, a powder coat method involving the heating and mixing with the resin powder may be properly employed.
In the invention, a two-component developer comprising a toner and a carrier can be used. Since as the carrier of the invention there is used a coating resin containing a carboxyl group-containing monomer, the carboxyl group, which is an anionic group, enhances the negative electrostatic property of the carrier, deteriorating the chargeability thereof with respect to the negative toner. In the invention, when the foregoing carrier is used, the carboxyl group-containing monomer can be used as binder resin for toner to improve the negative electrostatic property of the toner and hence maintain the chargeability of the developer. As the carboxyl group-containing monomer to be used as toner binder resin there may be used one exemplified as carrier coating resin. The carboxyl group-containing monomer may have the same or different structure from the carrier coating resin. So far as the carboxyl group is contained, the same effect can be exerted.
The particulate toner of the invention preferably has a shape factor SF1 of from 100 to 135. In order to prepare such a toner, a wet process is preferably used. Examples of such a wet process include (1) an emulsion polymerization coagulation process which comprises mixing a particulate resin dispersion obtained by the emulsion polymerization of polymerizable monomers of binder resin, a colorant dispersion, a release agent dispersion, and optionally a dispersion of charge controller, etc. to form coagulated particles, and then heating the coagulated particles so that they are coalesced to obtain a particulate toner, (2) a suspension polymerization process which comprises polymerizing a solution of a polymerizable monomer for obtaining a binder resin, a colorant, a release agent, and optionally a charge controller, etc. in the form of suspension in an aqueous solvent to obtain a particulate toner, and (3) a solution suspension process which comprises suspending a solution of a binder resin, a colorant, a release agent, and optionally a charge controller, etc. in an aqueous solvent to cause granulation. Optimum among these wet processes is emulsion polymerization coagulation process.
Examples of the binder resin to be used in the toner of the invention include homopolymer and copolymers of styrenes such as styrene and chlorostyrene, monoolefins such as ethylene, propylene, butylene and isoprene, vinylesters such as vinyl acetate, vinyl propionate, vinyl benzoate and vinyl butyrate, unsaturated carboxylic acids such as (meth)acrylic acid, vinylacetic acid, allylacetic acid and 10-undecenic acid, styrene derivatives such as carboxylstyrene, xcex1-methylene aliphatic monocarboxylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, dodecyl acrylate, octyl acrylate, phenyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate and dodecyl methacrylate, vinyl ethers such as vinyl methyl ether, vinyl ethyl ether and vinyl butyl ether, and vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone and vinyl isopropyl ketone. Particularly representative examples of the binder resin include polystyrene, styrene-acrylic acid alkyl copolymer, styrene-methacrylic acid alkyl copolymer, styrene-acrylonitrile copolymer, styrene-butadiene copolymer, styrene-maleic anhydride copolymer, polyethylene, and polypropylene. Further examples of the binder resin include polyester, polyurethane, epoxy resin, silicone resin, polyamide, modified rosin, and paraffin wax.
Representative examples of the colorant to be incorporated in the toner of the invention include magnetic powder such as magnetite and ferrite powder, carbon black, aniline black, chalcoyl blue, chrome yellow, ultramarine blue, Du Pont oil red, quinoline yellow, methylene blue chloride, phthalocyanine blue, malachite green oxalate, lamp black, rose bengale, C.I. pigment red 48:1, C.I. pigment red 122, C.I. pigment red 57:1, C.I. pigment yellow 97, C.I. pigment yellow 17, C.I. pigment yellow 12, C.I. pigment yellow 128, C.I. pigment yellow 151, C.I. pigment yellow 155, C.I. pigment yellow 173, C.I. pigment yellow 180, C.I. pigment yellow 185, C.I. pigment blue 15:1, and C.I. pigment blue 15:3.
Representative examples of the release agent to be incorporated in the toner of the invention include low molecular polyethylene, low molecular polypropylene, Fischer-Tropsch wax, montan wax, carnauba wax, rice wax, and candelilla wax.
The toner for the development of electrostatic latent image of the invention may comprise a charge controller incorporated therein as necessary. As the charge controller there may be used a known charge controller. In practice, however, azo-based metal complex compounds, metal complex compounds of salicylic acid or resin type charge controllers containing a polar group may be used. Preferably, a material which can difficultly be dissolved in water is preferably used from the standpoint of control over ionic strength and reduction of pollution by waste water. The toner of the invention may be a magnetic toner containing a magnetic material or a non-magnetic toner free of magnetic material.
These particulate toners may a magnetic material powder incorporated therein besides additives such as known charge controller, fluidity improver and fixing aid as necessary. The particle diameter of the toner tends to decrease as the image quality improves. The average particle diameter of the toner is from 2 to 12 xcexcm, preferably from 5 to 10 xcexcm.
The magnetic sleeve for carrying developer to be used herein may be made of a known magnetic material. In practice, however, the surface of the sleeve can be provided with a fine unevenness having Rz of from 15 to 25 xcexcm and Ra of from 1 to 5 xcexcm. In this arrangement, the developer can be provided with desired conveyance stability, the carrier can be prevented from flying, and a good image free from defects can be formed. Most commonly used sleeves have Rz of not greater than 10 xcexcm. When a carrier having a small diameter, a carrier having a small shape factor (shape factor SF1 of not greater than 125), a toner having a small diameter and a toner having a small shape factor (shape factor SF1 of not greater than 135) are used singly or in combination, the resulting developer exhibits an unstable conveyability. Therefore, in the case where these developers are used, the use of the foregoing magnetic sleeve is effective for the maximum accomplishment of the properties thereof.
The method for forming an image of the invention comprises a step of forming an electrostatic latent image on an electrostatic latent image holding member, a step of developing the electrostatic latent image with a developer on a developer holding member to form a toner image, a step of transferring the toner image to a transfer material, and a step of fixing the toner image on the transfer material, characterized in that as the developer there is used one containing the foregoing carrier, whereby the environmental dependence can be suppressed to form a good image.